The simple answer is 'crustal thickness'. The two crustal plates of India and Eurasia are very different in character. The Indian plate is thin, the Eurasian plate is much thicker and more rigid. Both plates are so buoyant with respect to the underlying mantle that neither can subduct in the normal sense. But India is moving so fast that something has to give. The Indian plate is buckling a bit, but mostly being under=plated at low angle as the Eurasian plate rides over it - as it did is such spectacular fashion during last year's Nepalese earthquake.

PS the main continent-continent crunch occurred some 40 to 20 years ago.

The simple answer is 'crustal thickness'. The two crustal plates of India and Eurasia are very different in character. The Indian plate is thin, the Eurasian plate is much thicker and more rigid. Both plates are so buoyant with respect to the underlying mantle that neither can subduct in the normal sense. But India is moving so fast that something has to give. The Indian plate is buckling a bit, but mostly being under=plated at low angle as the Eurasian plate rides over it - as it did is such spectacular fashion during last year's Nepalese earthquake.

PS the main continent-continent crunch occurred some 40 to 20 million years ago.